This invention relates to a pulse width modulated driver, and particularly, to a high side pulse width modulated driver for driving a load at a reduced voltage level.
Automotive manufacturers are currently designing automotive vehicles having 42 Vdc electrical systems in addition to conventional 12 Vdc electrical systems. Several advantages are obtained by utilizing 42 Vdc electrical systems. In particular, the 42 Vdc electrical systems may utilize a smaller gauge (i.e., diameter) of wirexe2x80x94as compared with 12 Vdc electrical systemsxe2x80x94because the increased voltage allows for a lower current flow through the wire. The reduced wire gauge also results in reduced weight and wire cost for an automotive vehicle.
A problem exists, however, with using a 42 Vdc supply voltage to drive a conventional halogen bulb in an automotive headlamp assembly. In particular, the conventional halogen bulbs (typically used in 12 Vdc electrical systems) have filaments that burn out relatively quickly when 42 Vdc is applied across the filaments. Specially designed halogen bulbs with longer and/or thinner filaments would need to be designed for the increased voltage level. However, these specially designed halogen bulbs would be more expensive and less robust as conventional halogen bulbs and would thus increase the cost of automotive vehicles and warranty.
When utilizing 42 Vdc electrical systems, pulse width modulated drivers have been utilized to drive the halogen bulbs at reduced voltage levels. Referring to FIG. 1, a conventional control circuit 10 using a conventional pulse width modulated driver 13 is shown. The driver 13 is utilized to drive a load 17, such as a halogen bulb. The driver 13 includes terminals 14, 15, 16. As illustrated, the driver 13 is connected to the normally-open switch 12 via the terminal 14. The driver 13 receives the supply voltage VCC, which may be 42 Vdc, when the switch 12 is closed. The driver 13 is further connected to a system ground via a terminal 15. Finally, the driver 13 is connected to the load 17 via the terminal 16. During operation, the driver 13 generates an output voltage that is applied to the load 17.
A disadvantage of the conventional driver 13 is that a separate ground line connected to the terminal 15 is required for its operation. When the driver 13 is located in an automotive vehicle at a relatively large distance from the system ground, a line must nevertheless be routed between the terminal 15 and the system ground. One skilled in the art will recognize that when a plurality of drivers 13 are utilized in an automotive vehicle, an additional ground line for each driver 13 will substantially increase the manufacturing costs of an automotive vehicle. Further, the amount of space utilized by the ground lines will decrease the amount of space available for additional automotive components.
There is thus a need for a pulse width modulated driver and a method related thereto that minimizes or reduces one or more of the above-mentioned deficiencies.
The present invention provides a high side pulse width modulated driver for driving a load, such as a halogen bulb.
The pulse width modulated driver in accordance with the present invention includes a switch selectively opening and closing to apply a pulse width modulated signal to the load. In particular, the switch opens and closes responsive to a switch control signal. The switch may comprise a transistor, and in particular, may comprise a MOSFET transistor. The driver further includes a duty cycle control circuit that generates the switch control signal when an input operational voltage is above a threshold voltage level. The duty cycle control circuit includes an oscillator. Finally, the driver includes an energy storage circuit for storing electrical energy when the switch is open to maintain the input operational voltage above the threshold voltage level. During operation, the inventive driver may receive a 42 Vdc supply voltage and generate an output voltage of approximately 12 Vdc (RMS) for driving a conventional halogen bulb.
A method for energizing a load, such as a halogen bulb, in accordance with the present invention is also provided. To implement the method, the load and a switch are connected in series between a supply voltage and system ground. The inventive method includes selectively opening and closing the switch to apply a pulse width modulated signal to the load responsive to a switch control signal. The method further includes generating the switch control signal using a duty cycle control circuit when the input operational voltage is above a threshold voltage level. Finally, the method includes charging a capacitor when the switch is open to maintain the input operational voltage above the threshold voltage level.
The pulse width modulated driver and the method related thereto in accordance with the present invention represents a significant improvement over conventional drivers and methods. In particular, when the inventive driver is utilized as a high side voltage driver (i.e., when the load is connected between the driver and system ground), the driver does not require a separate electrical line connected to system ground. In other words, the driver operates at a floating voltage potential with respect to system ground. One skilled in the art will recognize that an automotive vehicle may have a plurality of loads, such as halogen bulbs, each being controlled by a dedicated driver. Since each inventive driver can operate without a separate ground line, the number of electrical lines in an automotive vehicle may be greatly reduced. As a result, the inventive driver allows for decreased wiring and manufacturing costs associated with an automotive electrical system. Further, the inventive driver allows for decreased wiring space being utilized in an automotive vehicle since a ground line is not needed. Thus, an increased amount of space is available for other automotive components. Further, because the inventive driver may generate an output voltage of approximately 12 Vdc (RMS), the driver may control a conventional halogen bulb without decreasing the operational life of the bulb.
These and other features and advantages of this invention will become apparent to one skilled in the art from the following detailed description and the accompanying drawings illustrating features of this invention by way of example.